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Ask the Weather Guys: What is the difference between mist and fog?

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Sam Koblenski holds an umbrella for son Ian while he photographs fog over Lake Monona at Monona Terrace in Madison, Wis. The two were out photographing Frank Lloyd Wright architecture as a homework assignment in March.

Q: What is the difference between mist and fog?

A: Both a mist and a fog are water droplets suspended in the atmosphere in the vicinity of the earth’s surface that affect visibility.

They both differ from a cloud only in that the base of a fog or a mist is at the earth’s surface, while a cloud’s is above the surface.

The difference between a mist and a fog is associated with the atmospheric visibility. A fog and a mist are both composed of microscopic water droplets or wet hygroscopic particles suspended in the air. Particles cause light to be refracted and reflected in many directions, reducing visibility.

By international definition, a fog reduces visibility to below 1 kilometer (5/8 of a mile), while a mist occurs when the visibility at the earth’s surface is greater than 1 kilometer. These visibility observations are made at ground or sea level.

A fog is denser and thicker than a mist. Consequently, it is more difficult to see through a fog than a mist. A mist dissipates more quickly than a fog. In addition, the term “mist” is used in weather reports when the corresponding relative humidity is between 95% to 100%.

Haze is a related but slightly different phenomenon. Haze is a suspension of extremely small particles in the air, reducing visibility by scattering light like a fog and a mist. Haze formation is caused by the presence of an abundance of condensation nuclei which may grow in size, due to a variety of causes, and become a mist, a fog or a cloud.

"Weather Guys" Steve Ackerman and Jonathan Martin are professors in the University of Wisconsin-Madison department of atmospheric and oceanic sciences.

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